Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) as II-VI compound semiconductors are higher in radiation detection sensitivity and less susceptible to thermal noise than silicon (Si) and germanium (Ge), and hence are used for radiation detectors which operate at room temperature without separately requiring any cooling mechanisms. A radiation detector is formed by connecting, to a detection circuit, a radiation detection element having metal electrodes formed on the two surfaces of a substrate formed from a single crystal of CdTe or CdZnTe. The radiation detector converts the quantity of electrons emitted from the radiation detection element upon reception of radiation into a current and amplifies it by using the detection circuit, thereby detecting the radiation.
Vacuum deposition, plating, and the like are known as conventional methods of forming metal electrodes to be connected to circuits on the surfaces of CdTe substrates and CdZnTe substrates. Recently, electroless plating has been frequently used, which forms gold electrodes by immersing a CdZnTe substrate in a plating solution containing a gold compound such as chloroauric acid and depositing gold alone or a gold alloy at predetermined portions on the substrate surfaces (see Japanese Patent Application Laid-Open No. 2003-142673, Japanese Patent Application Laid-Open No. 2001-177141, Japanese Patent Application Laid-Open No. H08-125203, Japanese Patent Application Laid-Open No. H07-038132, Japanese Patent Application Laid-Open No. H03-248578 and Japanese Patent Application Laid-Open No. H03-201487).